The natural disaster has drawn attention to deeper political and financial inequalities between Puerto Rico — a U.S. territory — and U.S. states such as Florida and Texas, which are having an easier time returning to normal after their recent hurricane experiences. Unfortunately, there could be more trouble ahead, in the form of tiny tropical mosquitoes. Experts say the combination of natural disasters and persistent socioeconomic inequality creates an environment where mosquito-borne diseases — such as dengue, chikungunya and Zika — can spread.

All those diseases exist in Puerto Rico in the background of everyday life, occasionally flaring up into full-blown epidemics. Dengue, a virus that causes fever and joint pain, was diagnosed in 174 people in Puerto Rico in 2016 and none in the first half of 2017. But in bad years — 1994, 1998, 2007 and 2010, among them — it has infected more than 10,000. The same is true of other mosquito-borne diseases on the island. Zika, infamously, was epidemic in 2016, with more than 40,000 people diagnosed in Puerto Rico. By this June, though, cases of the disease had fallen to nearly nothing, and the epidemic was declared over.

It’s not always clear what factors make the difference between a year in which mosquito-borne disease is negligible and one in which it’s epidemic. But hurricanes alone aren’t necessarily big predictors, said Ben Beard, deputy director of the Centers for Disease Control and Prevention’s Division of Vector-Borne Diseases. The floods and winds that come with a storm kill mosquitoes and wash away their breeding grounds, and it’s not uncommon for a big hurricane to disrupt an outbreak in progress by temporarily cutting the local mosquito population off at the knees. But that effect is short-lived. “Within a week or so, you tend to see the situation come back to where it was before the storm,” he said. “Several weeks after that, you’ll see some increase.”

At that point, it starts to matter how a society has weathered the storm and how quickly it is recovering. The longer people live without solid roofs, intact window screens and air conditioning — and the longer they’re forced to spend large amounts of time outdoors rebuilding — the more likely it is that a storm will, indirectly, bring people and insects together.

Take, for instance, the connections between Hurricane Katrina and West Nile virus. This virus, carried by mosquitoes, already existed in Louisiana and Mississippi. But a 2008 study by Tulane University found that, in the weeks after Katrina passed through, hurricane-affected counties saw a two-fold increase in cases. Meanwhile, in counties that avoided the worst of the storm, cases of West Nile either went down or stayed the same. The increase was probably partially due to a burst of mosquito breeding in stagnant pools the storm left behind, said Peter Hotez, dean of the National School of Tropical Medicine at Baylor College of Medicine.

But mosquitoes don’t create an outbreak by themselves. To spread disease, you need people coming into contact with those mosquitoes. As the Tulane study noted, tens of thousands of hurricane survivors spent weeks in damaged homes or outside, waiting to be evacuated. The storm gave mosquitoes breeding grounds. Political disorder ensured that those new insects had access to humans.

Socioeconomic inequality — and the quality-of-life differences it creates — can have a big impact on who contracts mosquito-borne disease, even in the absence of a natural disaster, said Samuel Scarpino, a network science professor at Northeastern University who studies disease surveillance.

But it was the Mexican town that had much higher rates of residents whose blood tested positive for dengue exposure. Researchers connected that difference to disparities in living conditions between the two Laredos. Texans were much more likely to have central air conditioning, intact window screens and other little luxuries that created a barrier between them and the local mosquitoes.

Socioeconomic factors affect who gets dengue

A Texas town and its Mexican neighbor have very different disease outcomes

*This reflects the Breteau Index, a standard used by the World Health Organization for mosquito surveillance. It is not a percentage but instead records the number of water-filled containers found to be positive for mosquito larvae.

Source: Emerging Infectious Diseases journal

Findings like this have big implications for Puerto Rico, Scarpino said. The loss of electricity on most of the island could mean that most Puerto Ricans — even those who escaped the worst of the storm — will spend the next few months with no air conditioning. Instead, like the citizens of Nuevo Laredo, they’ll rely on windows to cool homes, schools, offices and government buildings — windows that are likely to have screens damaged by the storm.

What’s more, Scarpino told me, the loss of electricity and other forms of infrastructure is also likely to affect Puerto Rico’s mosquito-borne disease surveillance system. Scarpino recently published research analyzing that system, and he said it was one of the most impressive in the world because of its mosquito control and capture, high rates of disease testing and reporting, and molecular diagnostics that allow for quick, accurate test results. But those things rely on infrastructure lost to the storm: passable roads for trapping and collecting mosquitoes; intact hospitals and money for people to visit when they have fevers; electricity to power the diagnostic laboratories.

In other words, the impact of a hurricane isn’t just about the storm, it’s also about the place it hits. Puerto Rico’s political status, and its long-running economic and infrastructure crisis, could put its residents at risk of health problems that a U.S. state with a stronger economy wouldn’t have to worry about. And that’s not a huge surprise. Ultimately, Scarpino told me, this fits squarely into what we know about how diseases, in general, spread: “The biggest predictors are inequality and socioeconomic status.”

CORRECTION (Sept. 28, 10:35 a.m.): An earlier version of this article incorrectly identified the institution that Peter Hotez works for. He is dean of the National School of Tropical Medicine at Baylor College of Medicine, not Baylor University.

CORRECTION (Sept. 28, 12:52 p.m.): An earlier version of this story included an outdated title for Samuel Scarpino. He is a network science professor at Northeastern University, not a math professor at the University of Vermont.

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